It is well known that linear polyethylene may be prepared by the polymerization of ethylene (optionally with one or more olefins or diolefins such as butene, hexene, octene or hexadiene) using a "Ziegler" catalyst system which comprises a transition metal compound (such as a titanium halide or avanadium halide and an aluminum alkyl). Polyethylene produced in this manner, particularly "linear low density polyethylene", is in widespread commercial use. However, the conventional linear low density polyethylene ("lldpe") made with Ziegler catalysts suffers from a number of deficiencies. Most notably, conventional lldpe is a heterogeneous product which contains a small fraction of low molecular weight wax and a comparatively large amount of very high molecular weight homopolymer. The heterogeneous nature of these polymers generally detracts from the physical properties made from them.
Accordingly, a great deal of effort has been directed towards the preparation of "homogeneous" lldpe resins which mitigate this problem. In particular, it is now well known to those skilled in the art that so-called "metallocene" catalysts may be used to produce homogeneous lldpe resin. These homogeneous resins are, however, not without problems. Most notably, these homogeneous resins typically have a narrow molecular weight distribution and are difficult to "process" or convert into finished polyethylene products. Thus, efforts to improve the processability of homogeneous polyethylene resin by broadening the molecular weight distribution have been made and are disclosed, for example in U.S. Pat. No. 4,530,914; 4,701,432; 4,935,474; 4,937,299; 5,124,418 and 5,183,867.
In copending and commonly assigned patent applications there are disclosed certain phosphinimine catalysts which may be used to produce homogeneous polyethylene.